Giá trị biểu thức $\dfrac{1}{2} - 2.\left[ {\dfrac{1}{3} - \left( {\dfrac{{ - 5}}{4} + \dfrac{1}{3}} \right)} \right] + 0,25$ là:

$\begin{array}{l}\dfrac{1}{2} - 2.\left[ {\dfrac{1}{3} - \left( {\dfrac{{ - 5}}{4} + \dfrac{1}{3}} \right)} \right] + 0,25\\ = \dfrac{1}{2} - 2.\left[ {\dfrac{1}{3} - \left( {\dfrac{{ - 15}}{{12}} + \dfrac{4}{{12}}} \right)} \right] + \dfrac{1}{4}\\ = \dfrac{1}{2} - 2.\left[ {\dfrac{1}{3} - \left( {\dfrac{{ - 11}}{{12}}} \right)} \right] + \dfrac{1}{4}\\ = \dfrac{1}{2} - 2.\left[ {\dfrac{4}{{12}} + \dfrac{{11}}{{12}}} \right] + \dfrac{1}{4}\\ = \dfrac{1}{2} - 2.\dfrac{{15}}{{12}} + \dfrac{1}{4}\\ = \dfrac{1}{2} - \dfrac{{30}}{{12}} + \dfrac{1}{4}\\ = \dfrac{6}{{12}} - \dfrac{{30}}{{12}} + \dfrac{3}{{12}}\\ = \dfrac{{ - 21}}{{12}} = \dfrac{{ - 7}}{4}\end{array}$